1. Field of the Invention
The present invention is related to an optimized filter parameters design for digital IF programmable downconverter, and more particularly, to an optimized filter parameters design for a single channel digital IF programmable downconverter meeting input specifications and hardware constraints.
2. Description of the Prior Art
Software radio is a technique using software to reconfigure radio equipment to integrate different communication standards and to provide different communication services. The basic concept is to design the analog-to-digital converter (ADC) as close to the antenna modules as possible to digitize the received signal from antenna for further processing.
Due to the rapid development of broadband ADC technique and high-speed digital signal processing, it is now viable to sample and process the digital signal at intermediate frequency (IF) band. Digital IF technique plays an important role in software radio system because it can provide greater flexibility and higher efficiency compared to traditional analog circuit in specifications such as attenuation, selectivity and stability. A digital IF circuit in a typical software radio system comprises an ADC for sampling IF signal, a numerically controlled oscillator (NCO) for downconverting a digital IF signal into a baseband signal and a digital IF programmable downconverter for downconverting and filtering. Because of the high IF sampling rate, digital IF programmable downconverter is used to reduce the sampling rate and to filter out unwanted frequency bands.
Two major manufacturers, Intersil and GRAYCHIP, are providing hardware implemented with digital IF technique. The digital IF programmable downconverter products currently in the market are GRAYCHIP's GC4014 and GC4016 digital downconverters, and Intersil's HSP50016, HSP50214, HSP50216 and ISL50216 digital downconverters. Intersil's product family provides better control capability and programmability. HSP50016 digital downconverter is widely adopted since 1994, HSP50214 programmable digital downconverter rolled out in 1997, is highly programmable and suitable for narrow-band applications. Moreover, based on HSP50214 programmable digital downconverter, HSP50216 and ISL5216 digital downconverter are multi-channel products configured to use in wide-band communication systems, however, they are not related to the present invention.
A typical single channel digital IF programmable downconverter comprises four stages, including high speed down-sampling stage, spectral shaping stage, rate matching stage and oversampling stage. The high speed down-sampling stage uses simple Decimation filters, such as CIC (Cascaded Integrator-Comb) filter and Halfband filter for down-sampling. In the spectral shaping stage, one or more programmable FIR filters are used to re-shape and filter the output signal in digital downconverter, therefore the sampling rate of the input signal should conform to Nyquist sampling theory to get as many available taps of programmable FIR filter as possible. In the rate matching stage, fractional decimation filter or re-sampling FIR filter is used to achieve a non-integer decimation, wherein the output sampling rate is able to be adapted to user-specified sampling rate. The oversampling stage consists of a few interpolation filters, the main function of the oversampling stage is to enable programmable FIR filters in the spectral shaping stage to operate at a lower sampling rate to increase the available number of taps of programmable FIR filter.
According to the above-mentioned introduction of typical single channel digital IF programmable downconverter, it is difficult to provide an optimal design based on different types of filters.
In view of the above-described difficulty and complexity in designing a typical single channel digital IF programmable downconverter, after years of constant effort in research, the inventor of this invention has consequently developed and proposed an optimized filter parameters design for digital IF programmable downconverter